Recently, since highly precise and compact digital cameras, camera-equipped mobile-phones, and the like have been popularized, demands for weight saving and miniaturization of optical systems have been rapidly increased. In order to meet these demands, an optical design using a highly functional glass aspheric lens becomes the mainstream. In particular, a large-aperture aspheric lens using a glass showing a high refractive index and a low dispersion characteristic is important for the optical design.
Moreover, as a process for producing an aspheric lens, a precision press molding method which does not require a polishing step becomes mainstream in view of productivity and production costs. The precision press molding method is roughly classified into a direct press method in which a molten glass is directly dropped into a mold and pressed as it is and a reheat press method in which a glass preform having predetermined mass and shape is formed by dropping or the like from a molten glass (hereinafter the step is referred to as glass preform molding) and the resulting glass preform is placed in a mold and subjected to reheating and press molding. In the latter reheat press method, press moldability is important but it is also important to make a high-precision glass preform (glass preform moldability).
As glasses showing a high refractive index and a low dispersion characteristic, glasses containing B2O3—La2O3 as main components are known. However, in these glasses, an improvement in chemical durability, devitrification resistance, or press moldability is emphasized, but the glass preform moldability, i.e., to have a low liquidus temperature and be less prone to devitrification (devitrification resistance), to have a proper viscosity for forming a predetermined shape (e.g., a viscosity at a liquidus temperature, hereinafter a liquidus temperature viscosity, of 5 to 15 dPa·s), and the like, is not always sufficient.
The glass preform for press molding is produced, as one example, by dropping a glass melt from a platinum nozzle having a diameter of several millimeters (1 to 10 mm). In order to produce such a glass preform precisely and homogeneously by hot molding, it is desirable that crystal bodies precipitating at a liquidus temperature or lower are single-species crystals.
A glass containing B2O3—La2O3 as main components sometimes produces a glass preform at around liquidus temperature in a mass production process. In that case, a part thereof is cooled to the liquidus temperature or lower and crystal bodies (devitrified substances) may sometimes precipitate. A glass melt is formed by melting the crystal bodies but, when a plurality kind of the crystal bodies exist, at the time when the crystal bodies are melted, the resulting glass melt has not always the same composition as that of the glass melt before the formation of the crystal bodies. Thus, the glass melt becomes heterogeneous, causing striae. Therefore, it is preferable that an optical glass for glass preform has a low liquidus temperature and crystal bodies precipitating at the liquidus temperature or lower are single-species crystals.
On the other hand, as optical glasses, lower press molding temperature results in more improved mold durability, shorter molding cycle and more increased productivity, so that a low press molding temperature is also required.
In order to solve the above problems, a glass containing Li2O in addition to B2O3 and La2O3 as main components is known but, since it contains a large amount of rare-earth elements such as La2O3, there is a problem that a stable glass is not obtained when it is intended to heighten the refractive index.
In order to solve the above problems, a glass containing B2O3—SiO2—La2O3—Gd2O3—ZnO—Li2O—ZrO2 as main components has been proposed in Patent Document 1 but there is not specifically shown a composition of a high-refractive-index glass having a refractive index of 1.79 or more in Examples and, in addition, there is a problem that the molding temperature is high.
Furthermore, there have been proposed optical glasses for mold press molding containing B2O3—SiO2—La2O3—ZnO—Li2O—ZrO2—Ta2O5 as main components and having nd of 1.84 or more, νd of 35 or more, and Tg of 630° C. or lower in Patent Documents 2, 3, and 4. However, in Examples of Patent Document 2, the liquidus temperature is not specifically shown and, in addition, a mol % fraction of the content of La2O3 to the total content of La2O3, Gd2O3, Y2O3, and Yb2O3 is 0.66 or less. Therefore, a plurality of crystal bodies precipitating at the liquidus temperature or lower precipitate, so that the optical glass is not always sufficient in view of the control of glass preform molding.
Moreover, the glasses of Patent Documents 3 and 4 excessively contains Ta2O5 and a plurality of crystal bodies precipitating at the liquidus temperature or lower precipitate, so that the glasses are not always sufficient in view of the control of glass preform moldability and, in addition, there is a problem in view of saving cost.
In addition to the above, the glass is preferably also excellent in chemical durability. Namely, when chemical durability is insufficient, there is a concern that a glass surface gets clouded only by washing the glass. The phenomenon that the glass surface gets clouded is called weathering of glass. The weathering which looks white is called white weathering and the weathering which looks blue is called blue weathering. In a glass having a high refractive index and a low dispersion property, the blue weathering sometimes becomes a problem. Moreover, when the chemical durability is insufficient, there is a concern that a mirror property is also decreased by grinding and polishing other than washing.
The chemical durability of the glass depends on the dissolution of light element ions such as alkali ions including a B3+ ion, an Li+ ion and alkaline earth ions including a Zn2+ ion, so that a balance of the contents of respective components becomes important, with considering optical characteristics together.    Patent Document 1: JP-A-2003-201143    Patent Document 2: JP-A-2003-267748    Patent Document 3: JP-A-2006-016293    Patent Document 4: JP-A-2006-016295